Replace constant class var with an immutable type #759

Job	Run time
isort	5s
mypy	32s
sdist	1m 18s
black	10s
test-docs	2m 22s
linux-wheels (x86_64, cp38-manylinux*)	2m 7s
parse_sdl (windows-latest, 2.0.14)	1m 7s
linux-wheels (x86_64, pp38-manylinux*)	2m 5s
parse_sdl (windows-latest, 2.0.16)	48s
build-macos (cp38-*_universal2)	5m 56s
parse_sdl (macos-latest, 2.0.14)	40s
linux-wheels (aarch64, cp38-manylinux*)	2m 6s
build-macos (cp38-*_x86_64)	4m 52s
build (ubuntu-latest, 3.8, x64)	4m 3s
build (ubuntu-latest, 3.9, x64)	3m 39s
build-macos (cp38-*_arm64)	3m 34s
linux-wheels (aarch64, pp38-manylinux*)	1m 49s
build (ubuntu-latest, pypy-3.8, x64)	5m 2s
build-macos (pp38-*)	3m 57s
parse_sdl (macos-latest, 2.0.16)	43s
build (windows-latest, 3.8, x64)	4m 38s
build (windows-latest, 3.9, x64)	4m 39s
build (windows-latest, pypy-3.8, x64)	6m 43s
build (windows-latest, 3.8, x86)	4m 58s
build (windows-latest, pypy-3.8, x86)	6m 26s
isolated (ubuntu-latest)	1m 58s
isolated (windows-latest)	1m 47s
publish	0s
	1h 18m 4s

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